a) Field of the Invention
This invention relates to an intake system construction for an internal combustion engine and a manufacturing process of an intake passage section of an internal combustion engine. These intake system construction and manufacturing process are suited especially for application to a stratified burning internal combustion engine.
b) Description of the Related Art
In the field of developments of internal combustion engines, research and development work have been conducted in recent years on so-called stratified burning internal combustion engines in each of which a vertical swirl, that is, a tumble flow is formed in a combustion chamber in an intake stroke to permit efficient combustion of fuel in a combustion chamber.
A description is now made about such a stratified burning internal combustion engine. This internal combustion engine is designed to efficiently perform combustion of fuel by forming a tumble flow in a cylinder in an intake stroke and maintaining the tumble flow to a later stage of a subsequent compression stroke, for example, as shown in FIG. 20.
FIG. 20 illustrates the construction of one cylinder of an internal combustion engine which has been designed to form tumble flows. In FIG. 20, there are depicted a cylinder 1A, a reciprocating piston (hereinafter referred to simply as the "piston") 2, a cylinder head 3, an intake port 4, an intake port open end 4A and a combustion chamber 7.
Described specifically, the combustion chamber 7 is formed between a top wall of the piston 2, which is fittedly inserted in the cylinder 1A, and a lower wall of the cylinder head 3. The intake port open end 4A is formed in the cylinder head 3 so that the intake port 4 opens to the combustion chamber 7 through the intake port open end 4A.
In diesel internal combustion engines, which primarily use diesel fuel or the like, it has been the practice to directly inject fuel into a combustion chamber and then to cause it to undergo autoignition by compressed air in the combustion chamber so that power is obtained.
In the meantime, a variety of in-cylinder injection gasoline engines of the type that fuel is directly injected into each combustion chamber to improve the responsibility of the engines have also been proposed.
A brief description will now be made about the specific construction of such an in-cylinder injection gasoline engine. For the in-cylinder injection gasoline engine, a spark plug is needed as ignition means. This spark plug is arranged in a combustion chamber. Further, an injector is disposed in a cylinder head, for example, on a side of an intake valve.
In such an in-cylinder injection internal combustion engine, it is also desired to improve the gas mileage of the engine by forming an eddy-like flow of inducted air in the combustion chamber and performing lean burn with fuel leaner than a stoichiometric air-fuel mixture.
A construction has therefore been proposed with a view to forming such an eddy-like flow of inducted air. According to this construction, an inducted air flow from the intake port is introduced in a direction as parallel as possible with a lower wall of the cylinder head and is then caused to advance downwardly along an inner peripheral wall of the cylinder so that an eddy-like inducted air flow is formed.
However, to form an eddy-like inducted air flow by using such a construction, it is necessary to arrange the intake port in a direction as parallel as possible with the lower wall of the cylinder head. In an in-cylinder injection internal combustion engine, no sufficient space is therefore retained for the arrangement of an injector.
It was hence contemplated--like the technique disclosed, for example, in Japanese Patent Application Laid-open (Kokai) No. HEI 6-146886--to form a strong vertical swirl (tumble flow) in a combustion chamber so that an engine can be stably operated even in a lean burn mode.
Such a technique is however rather insufficient for the formation of a vertical swirl of a high tumbling degree because an intake port is bent toward a center axis of an associated cylinder.
To cope with the above problem, a construction has been proposed as shown in FIG. 20, that is, an intake port is bent (or inclined) in a direction away from a central axis of an associated cylinder so that a vertical swirl of such a high tumbling degree as described above can be surely formed.
Even with the technique proposed immediately above, there is the problem that formation of a tumble flow of sufficient strength becomes difficult when the volume of the combustion engine increases to or beyond a certain value.
Namely, when the volume of the combustion chamber becomes large, a flow of intake air inducted into the cylinder becomes stronger. As illustrated in FIG. 20, upon entrance of the inducted air flow into a cylinder 1A from an intake port 4, the inducted air flow strikes a head portion 61a of an intake valve 61 so that the inducted air flow is bifurcated to both sides of the head portion 61a. Accordingly, the intake valve 61 itself becomes an induction resistance and hence acts as an adverse factor upon formation of a vertical swirl in the cylinder 1A.
In such a stratified burning internal combustion engine, it is desired to feed an inducted air flow from the intake port 4 into the combustion chamber 7 under minimized disturbance so that formation of the inducted air flow into a stratified tumble flow can be facilitated.
Incidentally, the intake port 4 is internally provided with the intake valve 61. A portion of an inducted air flow may therefore strike the stem portion 61b so that the inducted air flow may be disturbed. Specifically, when a portion of an inducted air flow strikes the stem portion 61b, vortices tend to occur in a flow behind the stem portion 61b, thereby forming a region where inducted air flows at a lower velocity.
The flow velocity is therefore lowered behind the stem portion 61b and a substantial loss takes place in the flow. This leads to the inconvenience that formation of a strong tumble flow in the combustion chamber 7 may be prevented. To form a strong tumble flow or, speaking in a still broader sense, not only to form a strong tumble flow but also to improve the efficiency of induction of air, it is required to suppress the occurrence of a disturbance to an inducted air flow by the stem portion 61b.
Integral formation of means as a raised portion extending into a flow passage from an inner peripheral wall of the intake port 4 for the suppression of the occurrence of a disturbance to an inducted air flow, however, involves the potential problem that configurational variations may take place upon casting, possibly resulting in occurrence of differences in the efficiency of air induction among individual intake ports.
Further, an intake port generally has a complex internal configuration. It is therefore not easy to form, in such an intake port, a raised portion in dimensions as preset. The formation of such a raised portion also involves the problem that greater man-hour hours of labor are needed.